The vast majority of digital cameras sold to the general consumer are based on complementary metal oxide semiconductor (CMOS) image sensors since CMOS image sensors may be manufactured in large volumes at low cost. CMOS image sensors are produced at the wafer-level on silicon substrates and the manufacturing involves appropriately doping spatially distinct areas of the wafer to produce local features having different types and degrees of doping, namely various degrees of p type doping (positive charge carriers) and various degrees of n type doping (negative charger carriers).
Each pixel of a CMOS image sensor has a photodiode for generating electrical charge in response to incident light. The photodiode is coupled with a set of transistors for reading out the electrical charge to generate an image signal and for resetting the pixel after readout. Typically, three or more transistors cooperate to read out the electrical charge and reset the pixel. The transistors are formed in/on the silicon substrate next to the photodiode in one or more “islands” separated from the surrounding components by shallow-trench isolation. Shallow-trench isolation is an etched trench filled with a dielectric.
Consumers have become accustomed to digital photography producing excellent image quality, and the demand for higher spatial resolution is ever increasing. To meet this demand, CMOS image sensors are produced with a very large number of pixels, which leads to a significant lateral resistance in the substrate between centrally located pixels and the perimeter of the pixel array. As a result, a peripheral ground contact does not provide a uniform ground plane for the entire pixel array. This problem may be overcome by placing a ground contact in each pixel.